Apple XNU: Clutch Scheduler

trueno | a day ago

interesting. id love an eclecticlight breakdown of this. they're one of the few if only that write anything worth reading on apple hardware, i once found a QOS/scheduler insight through those guys when I couldn't get my c/cpp project pinned to the cores I wanted on m-series. https://eclecticlight.co/m1-macs/

saagarjha | 18 hours ago

He doesn’t read code. Convert it to some log messages and he might explore it.

KerrAvon | 6 hours ago

Please take anything this fella writes with several grains of salt.

denotational | 6 hours ago

Please could you elaborate?

cadamsdotcom | a day ago

> The XNU kernel runs on a variety of platforms

This is fascinating, would love to know where it’s used! (Besides macOS)

electronsoup | a day ago

Perhaps they mean ISAs

xphos | 22 hours ago

Well x86 at one point, arm both the 32 and 64 bit versions. I think they had RISCV support in their source tree at one point but not really at a commercial level. It does cover a lot different levels of hardware though

wiml | 21 hours ago

PPC32/64 of course, and for a long time Darwin still contained remnants of its predecessor's support for SPARC, PA-RISC, and m68k.

kjs3 | 21 hours ago

Is mc68k or PPC still in there anywhere?

monocasa | 19 hours ago

I'm sure there's vestiges of them somewhere, but the underlying support (the architecture specific parts of the mach portion of the kernel) is gone for those archs.

mghackerlady | 8 hours ago

I wouldn't be surprised if they keep a minimal Power base maintained behind closed doors. It's how they managed to jump ship to intel so quickly, they never stopped maintaining NeXTSTEPs x86 port

monocasa | 2 hours ago

I would honestly be shocked if they were.

They've been making quite a few changes to the virtual memory code over the past decade, and keeping those vestigial arch's around is a pretty big maintenance burden. It'd probably be less work to just add the arch as if it were new when it's needed at this point since the kernel itself is pretty portable.

userbinator | 17 hours ago

Which Apple products run arm32 XNU? Their first Apple Silicon CPUs were already arm64.

dagmx | 16 hours ago

Well there were still the historical arm32 chips in their iOS devices, but until recently the watches were a cursed arm64_32 (or something like that) which is arm64 with 32 bit pointers iirc.

xphos | 8 hours ago

I should have just soureced this, They had PowerPC not RISCV in there source tree that was the X factor one. The Arm32 bit variant is closed sourced (leaked before) but was supported until IOS 11. XNU is really old almost 30 years! And before XNU there was the MACH kernel and the larger BSD tree it was built on which is an argument that it probably had a initial MIPs release too but I couldn't source the truth on that.

[1] https://en.wikipedia.org/wiki/XNU

skissane | 17 hours ago

Does Apple use macOS in servers in its datacentres? Or are they all Linux?

Surely at a minimum they need macOS for CI.

Apple does have one advantage here-they can legally grant themselves permission to run macOS internally on non-Apple hardware, and I don’t believe doing so legally obliges them to extend the same allowance to their customers.

But that might give them a reason to keep x86_64 alive for internal use, since that platform (still) gives you more options for server-class hardware than ARM does

jabwd | 13 hours ago

They do run Apple Silicon in data centers, so perhaps another custom version of Darwin + their system frameworks. It is hard to tell without some leaks :)

glhaynes | 6 hours ago

For Private Cloud Compute: “a new operating system: a hardened subset of the foundations of iOS and macOS tailored to support Large Language Model (LLM) inference workloads while presenting an extremely narrow attack surface.” https://security.apple.com/blog/private-cloud-compute/

therein | 13 hours ago

They use Ubuntu on x86-64 servers, at least for iCloud. Backends for iCloud, Photos and Backups etc. are written in Java.

Longhanks | 10 hours ago

Any sources or more information on that?

twalla | 7 hours ago

For the Java bit at least, this aligns with job descriptions I’ve seen and recruiter outreach I’ve received (long time ago though, maybe 5 years).

pjmlp | 4 hours ago

NeXT added a Java variant to WebObjects and it was for several years the main server side infrastructure, after being acquired by Apple.

Nowadays you can usually still find Java and JVM languages like Clojure (Apple Maps), on Apple's job ads.

How much of it is still Java based, no idea.

I imagine XCode Cloud has nothing to with it for example.

sillywalk | 4 hours ago

They moved their password monitoring service from Java to Swift.

https://news.ycombinator.com/item?id=44172166

https://www.swift.org/blog/swift-at-apple-migrating-the-pass...

therein | an hour ago

Unfortunately I am the source in this case. It is from having worked on them personally. :)

LoganDark | 22 hours ago

IIRC, Apple uses 'platform' to refer to an SoC integration. For example, M1, M2 and etc. are separate platforms. M5 in Vision Pro is a separate platform than M5 in MacBook Pro. I believe Apple's XNU does somewhat still support non-Apple Silicon as well though.

fragmede | 22 hours ago

Yeah they're was that whole x86 thing thru did for quite a while.

simonh | 11 hours ago

Twice, on the basis that NEXT used the same kernel and that ran on 68k and Intel when Apple bought them and later ported it for Power PC. When Steve Jobs went back to Apple, for a long time he ran NEXT on a Thinkpad.

mghackerlady | 8 hours ago

NeXTSTEP also ran on SPARC iirc

pjmlp | 4 hours ago

OpenSTEP actually.

mghackerlady | an hour ago

OpenSTEP had SPARC support, yes, but NeXTSTEPs last release had support for m68k, x86, and SPARC. 3.3 had support for PA-RISC

csb6 | a day ago

I believe it means Apple's other hardware platforms (phones, tablets, smart TVs, VR headsets, smartwatches)

LoganDark | 22 hours ago

It's used in iOS as well. iOS runs in some unexpected places, like for example Studio Display. Also, the Apple Lightning Digital AV Adapter runs Darwin (because RTKit didn't exist yet).

nxobject | 18 hours ago

And touchbars too, strangely enough.

LoganDark | 18 hours ago

For Intel platforms, the Touch Bar is driven by the trusted coprocessor (T1/T2), but that itself runs bridgeOS which indeed is Darwin/watchOS-based. With Apple Silicon I don't know if bridgeOS is still used; the SEP runs an L4.

asimovDev | 9 hours ago

that lightning AV adapter is crazy, iirc it creates an ethernet connection to airplay the display to the device

internet2000 | 18 hours ago

All of Apple's platforms.

almoni | 23 hours ago

Does this contribute to macOS's suitability for DAW applications or is that more the baked in low-latency audio drivers?

bigyabai | 23 hours ago

CoreAudio probably deserves most of the credit, there. Similar ASIO-style solutions like JACK, DirectSound and now Pipewire hit the sub-30ms mark without any big scheduler tweaks.

lukeh | 17 hours ago

IOKit was designed to support CoreAudio from the start, cc gvdl.

KerrAvon | 6 hours ago

CoreAudio was developed alongside xnu / IOKit for Mac OS X, so it’s kind of all of it. Apple had the opportunity to start fresh with a built-in super low-latency audio subsystem at the turn of the century, and they took it.

dcrazy | 22 hours ago

Audio actually runs on a dedicated realtime thread. This used to be scheduled differently, but nowadays it might be implemented by the FIXPRI bucket described in this document.

poige | 16 hours ago

Darwin had bunch of schedulers except this one: dualq, multiq, etc

In fact here's the one used in Sonoma: sysctl kern.sched -> edge

which seems to be an extension over "clutch":

https://github.com/apple-oss-distributions/xnu/blob/main/osf...

"… Single-cluster, symmetric (SMP) systems can run with just the Clutch policy, but multi-cluster, asymmetric (AMP) systems must further enable the Edge policy extension to Clutch in order to manage scheduling across the multiple CPU clusters. …"

dcrazy | 7 hours ago

Edge is also described in the linked document.

travisgriggs | 7 hours ago

Read through much of this. Definitely started feeling like “a picture might be worth 1,000+ words”.